A multi slice X-ray CT apparatus is generally configured, as shown in FIG. 3, to obtain projection data of an object 17 by irradiating X-ray beams of cone beam that are pyramid shape from an X-ray tube 8 and measuring X-ray after transmitting through the object by a detector 11 in which detection elements 18 are arranged two-dimensionally (in channel direction and row direction).
Also in single slice X-ray CT apparatus, projection data of the object 17 is obtained by irradiating X-ray beams of fan beam that is sector form beam from the X-ray tube 8 to the object 17 using the detector 11 in which detector elements are arranged in one row that is one-dimensionally (in channel direction), and measuring the X-ray after transmitting through the object 17.
In either case, projection data is obtained from multiple directions by rotating the X-ray tube 8 and the detector 11 which are facing each other around the object 17, and performing reconstruction filtering process for deblurring, then a tomgraphic image of the object 17 is reconstructed by back projection.
Projection data is obtained at discrete X-ray tube position (hereinafter, it is referred to as “view”), and the obtained projection data is referred to as “projection data in the relevant view”. The number of views per one rotation generally extends to several hundreds to several thousands. The operation for obtaining projection data of the view number necessary for reconstructing one tomographic image is referred to as a “scan”. Also, projection data for one view is formed by data for the number of channels times the number of rows of the detector 11 (for the single slice X-ray CT apparatus, it is calculated as in the case that the number of row=1) as previously described.
In the past, there have been X-ray CT apparatus, in order to perform scanning which satisfies an image SD (Standard Deviation) value necessary for image reconstruction, capable of performing scanning by calculating an elliptic cross section model of an object from scanogram projection data obtained by scanogram imaging in one direction, and calculating appropriate tube current value from the projected area of the elliptic cross section, aspect ratio of the elliptic cross section and the desired image SD value inputted by an operator. Hereinafter, in setting stage of scan planning, a function for inputting desired image quality index value (image SD value, etc.) within the scanning range and properly modulating an irradiation X-ray dose (scan dose) to satisfy the image quality index value thereof is referred to as a X-ray dose optimization function.
However, without knowledge of the suitable value as the desired image quality index value prior to scanning, actual acquisition of proper images can not be carried out even with the usage of the above-described X-ray dose optimization function.
To solve this problem, an X-ray CT apparatus for generating and displaying simulated images corresponding to the set scanning condition and the desired image noise index value (image SD value) is disclosed in Patent Document 1.
Patent Document 1: JP-A-2004-329661